1. Field of the Invention
The present invention relates to a correction method for a geomagnetic field sensor mounted on a vehicle.
2. Description of Relevant Art
The geomagnetic field sensor is a magnetometer for detecting the spot geomagnetic force in the form of a vector, and has been and is utilized as a bearing sensor for vehicle-mounted bearing meters for detecting the bearing of a vehicle to inform the driver of such bearing.
There has been disclosed in Japanese Patent Lay-Open Print No. 58-70114 (Apr. 26, 1983) a correction method for a geomagnetic field sensor of a vehicle-mounted bearing indication system, as described below with reference to the accompanying prior art drawings FIG. 6 to FIG. 10.
In FIG. 6, which is a schematic connection diagram of a bearing indication system for vehicles, the bearing indication system includes a geomagnetic field sensor 1 of the flux gate type of a well-known toroidal core design. Sensor 1 detects the spot geomagnetic force in the form of a radius vector A on a horizontal X-Y plane as orthogonal coordinates of which X and Y axes are fixed to be perpendicular to a main sensing direction D or the direction of the main sensing axis of the sensor 1 and to be directed in the main sensing direction D, respectively. The vector A consists of a pair of orthogonal components, i.e., an X component as X direction sine of the geomagnetic force vector A and a Y component as Y direction cosine thereof. Sensor 1 outputs the vector A in the form of a pair of voltages Vx', Vy' proportional to the X and Y components thereof, respectively. An amplifying circuit 2 is connected to the geomagnetic field sensor 1. The circuit 2 includes a pair of amplifiers 2a, 2b for amplifying the voltages Vx', Vy' to thereby output corresponding amplified voltages Vx, Vy,. A bearing meter 3 is connected, directly in a simple example, to the amplifying circuit 2. The meter 3 includes a bearing pointer 3a for indicating the bearing of a vehicle relative to the geomagnetic north N at each travelling spot in accordance with the amplified output voltages Vx, Vy. In an improved example, a signal processor (not shown) is interposed between the amplifying circuit 2 and the bearing meter 3. The processor is adapted to correct the output voltages Vx, Vy making use of the conception of a later-described offset point. In the bearing meter 3, the voltages Vx, Vy as directly input or corrected are combined with each other to generate a magnetic field in which the bearing pointer 3a, having secured thereto a permanent magnet (not shown), is caused to rotate by an angle of corresponding degrees, thererby indicating the bearing of the vehicle.
The bearing meter 3 has an input terminal a for receving the voltage Vx, and another input terminal b for receiving the voltage Vy. With an X-Y recorder or plotter 4 connected to the terminals a, b, when the geomagnetic field sensor 1 as put in a sufficiently uniform geomagnetic field is turned by one revolution in the inital X-Y plane, the X-Y recorder 4 has thereon a circular trace B (FIG. 7) drawn about the center thereof located at the origin O of Vx-Vy orthogonal coordinates of the recorder 4. The set of coordinates (Vx, Vy) constitutes a mapping in the X-Y plane of the geomagnetic force vector A, such that the axes of abscissa and ordinate Vx, Vy indicate the west-to-east and south-to-north directions, respectively, at any geographical spot of the sensor 1. When the main sensing direction D of the sensor 1 is put in any position relative to the geomagnetic north N in the X-Y plane or substantially to the absolute geomagnetic north, the recorder 4 has plotted on its Vx-Vy plane a point P in the same position as above relative to the geomagnetic north N as mapped on the Vx-Vy plane, thereby visualizing the variation of the geomagnetic field as detected by the sensor 1. Accordingly, with respect to a located vector C beginning at the origin O and ending at the plotted point P on the Vx-Vy plane, the direction of the vector C relative to the axis Vy represents the bearing of the sensor 1 relative to the absolute geomagnetic north.
As shown in FIG. 8, in a practical example, the above-described bearing indication system is mounted on a vehicle 5, with the main sensing direction D of the sensor 1 in coincidence with the direction F of the longitudinal centerline of the vehicle 5. When the vehicle 5 is driven to travel in a circle in a sufficiently uniform geomagnetic field, thus causing the geomagnetic field sensor 1 to turn by one revolution, the X-Y recorder plots on the Vx-Vy plane a circular trace B' (FIG. 9) about a center O' thereof which usually deviates from the origin O, due to the magnetization of a vehicle body of the vehicle 5. In other words, the vehicle 5 has generated by the body magnetization a local magnetic field representable by a located vector OO' beginning at the origin O and ending at the deviated center O'. In this respect, at any plotted point P on the Vx-Vy plane, the pointer 3a of the bearing meter 3 will indicate, as the apparent bearing of the vehicle 5, the same direction as a located vector C' beginning at the origin O and ending at the point P on the Vx-Vy plane, while the true bearing of the vehicle 5 should be indicated as the direction of a located vector O'P beginning at the deviated center O' and ending at the point P.
In the above-described bearing indication system, to avoid such influence of the local magnetic field due to the body magnetization of the vehicle 5 on the geomagnetic field sensor 1 mounted on the vehicle 5, the amplified output voltages Vx, Vy are corrected by the signal processor having applied thereto a correction method. Such method includes steps of driving the vehicle 5 in a circle, thereby unveiling the deviation of the detected geomagnetic field due to unneglectable body magnetization such as the crossing magentization of the vehicle 5, and correcting the output voltages Vx, Vy in consideration of such deviation. More particularly, in the correction method, by driving the vehicle 5 in a circle, there are obtained on the Vx-Vy plane a maximum value Vxmax in a set of Vx components and a minimum value Vxmin therein and likewise a maximum value Vymax in a set of Vy components and a minimum value Vymin therein, as shown in FIG. 10. The deviation is determined in terms of an offset point (Xoff, Yoff) by using a pair of expressions such that: EQU Xoff=1/2 (Vxmax+Vxmin) EQU Yoff=1/2 (Vymax+Vymin)
Then, the X and Y components Xoff, Yoff of the offset point are subtracted from the output voltages Vx, Vy, respectively, before these are input to the bearing meter 3, thereby indicating the bearing, i.e., the traveling direction, of the vehicle 5 relative to the geomagnetic north N at each traveling spot.
Incidentally, in the foregoing description, the origin O of the Vx-Vy coordinates is a point at which the sensor 1 has zero ouput and is easy to identify. Further, the identification of the extreme values Vxmax, Vxmin, Vymax, and Vymin and the results thereof, as well as the offset point (Xoff, Yoff) are processed by and stored in the signal processor. In the processor, the local magnetic field due to body magnetization or the vector OO' given in terms of an offset vector (Xoff, Yoff) is eliminated or componentwise deducted from the apparent geomagnetic field or the vector C', i.e., a parallel displacement vector (-Xoff, -Yoff) opposite in the direction to the offset vector (Xoff, Yoff) is added to the apparent geomagnetic force vector C'.
According to the conventional correction method, however, even after a vehicle equipped with an offset-corrected bearing indication system i.e., a conventional system having an offset point once stored in a signal processor, has traveled through a strong magnetic field, erroneous bearings may still undesirably be generated and displayed. For example, a strong magnetic field occurs when crossing an electric railroad, whereby the vehicle becomes varied in its magnetic field due to body magnetization. In such a situation the signal processor has a parallel displacement vector still associated with the stored (and now incorrect) offset point, and which parallel displacement vector is added as it has been to an apparent geomagnetic force vector at each traveling spot Thus, an erroneous bearing will be given on a bearing meter of the bearing indication system.
In this respect, such erroneous indication can be preventable by driving the vehicle to make a single turn, each time when having passed such strong magnetic field, to thereby update the offset point. However, the fact that it is considerably difficult for the driver to know from time to time the variation of the magnetic field due to body magnetization of the vehicle must be considered. Also, the driving maneuver of making the single turn for no more than the output correction of a geomagnetic field sensor is practically troublesome and inconvenient for the driver. Forcing the driver to make such correction from time to time would reduce the commercial value of the bearing indication system.
Moreover, according to the above-described correction method, in which the offset point is determined by way of what is called a maximum-minimum method from respective maximum and minimum values of output signals of the geomagnetic field sensor, when setting an offset point there are electromagnetic factors other than the vehicle body magnetization which must be considered, such as other vehicles in the vicinity, a geographically localized abnormal magnetic field, and a high-tension cable or transmission line. Thus, the offset point stored remains erroneous, giving an erroneous indication of the bearing of the vehicle, until a subsequent setting has a correct offset point computed to be stored.
The present invention has been achieved to effectively overcome such problems in the conventional correction method for geomagnetic field sensors.